The present invention relates to an alloy used for joining to cemented carbide useful for wear-resistant tools or cutting tools, and its composite material.
Cemented carbide comprising hard grains of carbides, nitrides or the like bound with a binding phase are useful for cutting tools, plastic working tools such as rolls and punches, and drilling tools for civil engineering, and mining purposes, because of their excellent hardness and wear resistance. However, grave problems with cemented carbide are that they cost much due to difficulty involved in machining, and that they have limited application because of susceptibility to damage due to their low toughness. Included in approaches to overcoming such difficulty or limitation are the brazing and welding, and mechanical fixing, e.g., screwing or shrink fitting of cemented carbide to a matrix material that is inexpensive and excellent in machinability, e.g., steel, thereby obtaining composite materials.
The mechanical fixing such as screwing, shrink fitting, and clamping entails extra work for the fixing of a hard-to-machine cemented carbide, resulting in an unavoidable limitation on its application in view of shape, and machining. When the brazing material is used, the brazing material or cemented carbide may often crack due to internal stress based on a difference in the coefficient of thermal expansion between the brazing material, cemented carbide, and matrix material. Another problem with the use of the brazing material is that it has a melting point lower than that of the cemented carbide or matrix material, and so it yields a composite material having a low heat-resisting temperature and, hence, a reduced strength.
Such a problem with the use of the brazing material may possibly be averted by joining the matrix material directly to the cemented carbide through diffusion joining. When, for instance, steel is diffusion-joined to a WC--Co type cemented carbide that is one of most representative cemented carbide, however, there is a problem that any sound junction is not obtained because of the generation of thermal stress due to a large thermal expansion difference, as in the case of the aforesaid brazing, and a brittle phase is formed in a diffusion layer. In the case of the joining of steel to a WC--Ni type cemented carbide, too, any sound junction is not obtained by diffusion joining.
Accordingly, it is an object of the present invention to provide a joining alloy which, even upon joined to a WC--Co or WC--Ni type cemented carbide, ensures high-enough joining strength with no formation of a brittle phase in the junction. Another object of the invention is to provide an inexpensive cemented carbide composite of high reliability, high strength and high performance, which comprises a cemented carbide-joining alloy-steel system.